Catalytic oxidation of carbon monoxide

ABSTRACT

A composition of matter comprising hydrotalcite and Pt and/or Pd has been prepared by a process comprising the steps of impregnating hydrotalcite with Pt and/or Pd, followed by calcining and, preferably treatment with a reducing gas. The above composition of matter is used as catalyst in a process for oxidizing CO to CO 2 , e.g., in a CO 2  laser.

BACKGROUND OF THE INVENTION

This invention relates to the oxidation of carbon monoxide to carbondioxide. In another aspect, this invention relates to the catalyticoxidation of carbon monoxide, in particular under conditions suitablefor laser applications. In a further aspect, this invention relates toan effective CO oxidation catalyst composition. In still another aspect,this invention relates to a process for preparing a CO oxidationcatalyst composition.

The use of catalysts for the oxidation of carbon monoxide to carbondioxide by reaction with oxygen, in particular at low temperature, is ofmuch interest, e.g., in breathing masks designed to removed CO frominhaled air, and in CO₂ lasers for combining CO and O₂ formed bydissociation of CO₂ during discharge. In the latter application, thepresence of O₂ is most undesirable because it can cause a breakdown ofthe electrical field in the laser cavity. Several patents, such as U.S.Pat. Nos. 4,490,482 and 4,639,432, disclose compositions useful as COoxidation catalysts in CO₂ laser applications. However, there is an everpresent need to develop new, effective CO oxidation catalystcompositions and/or improved processes for preparing effective COoxidation catalyst compositions.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a composition of matterwhich is effective as a catalyst for the oxidation of carbon monoxidewith free oxygen. It is another object of this invention to provide aprocess for preparing a composition of matter which is effective as acatalyst for the oxidation of carbon monoxide. It is a further object ofthis invention to provide an effective process for catalyticallyoxidizing carbon monoxide. Other objects and advantages will be apparentfrom the detailed description and the claims.

In accordance with this invention, a process for preparing a compositionof matter, which is useful and effective as a catalyst composition forthe oxidation of carbon monoxide (CO) by reaction with free oxygen (O₂),comprises the steps of:

(a) contacting (preferably impregnating)

a support material comprising (preferably consisting essentially of)hydrotalcite with

a solution having a pH of at least about 5 (preferably about 6-8) andcomprising at least one dissolved compound of at least one noble metalselected from the group consisting of platinum and palladium (preferablyPt); and

(b) heating the material obtained in step (a) under such conditions asto substantially dry said material obtained in step (a) and to at leastpartially (preferably substantially) convert said at least one compoundof Pt and/or Pd to at least one substance selected from the groupconsisting of oxides of Pt, oxides of Pd, Pt metal and Pd metal.

Preferably, the preparation process comprises the additional step of:

(c) heating the material obtained in step (b) in a reducing gasatmosphere, preferably a free hydrogen containing gas or a carbonmonoxide containing gas, more preferably a stream of hydrogen gas(substantially pure H₂), under such conditions as to activate saidmaterial obtained in step (b), i.e., to make the material obtained instep (b) more active as a catalyst for CO oxidation by reaction with O₂.

In a preferred embodiment, heating step (c) is carried out at atemperature in the range of from about 50° to about 450° C. In a furtherpreferred embodiment, the solution used in step (a) additionallycomprises at least one dissolved compound of iron, which is at leastpartially (preferably substantially) converted to iron oxide in step(b).

Also in accordance with this invention, there is provided a compositionof matter (useful and effective as a catalyst composition for theoxidation of CO with O₂) comprising (i) a support material comprising(preferably consisting essentially of) hydrotalcite and (ii) at leastone noble metal selected from the group consisting of Pt and Pd; saidcomposition of matter having been prepared by the process describedabove comprising steps (a) and (b), and preferably also (c). Preferably,said composition of matter further comprises (iii) iron oxide. In a morepreferred embodiment, the composition of matter consists essentially ofcomponents (i), (ii) and (iii).

Further in accordance with this invention, a process for oxidizingcarbon monoxide comprises contacting a gas mixture comprising CO and O₂with a catalyst composition comprising components (i) and (ii), asdefined above, said catalyst composition having been prepared by aprocess comprising steps (a), (b) and (c), as defined above; under suchcontacting conditions as to at least partially (preferablysubstantially) convert CO and O₂ to CO₂.

Preferably, in the CO oxidation process of this invention, the catalystcomposition of this invention (described above) additionally comprisesat least one iron compound (preferably, FeO and/or Fe₂ O₃ and/or Fe₃O₄). In a preferred embodiment, the CO oxidation process of thisinvention is carried out at a temperature in the range of about 0° toabout 100° C.

DETAILED DESCRIPTION OF THE INVENTION

Any hydrotalcite containing material can be used as the supportmaterial. The hydrotalcite can be naturally occurring or synthetichydrotalcite or a synthetic hydrotalcite-like compound or mixturesthereof. Presently preferred are the hydrotalcite materials described inU.S. Pat. Nos. 4,347,353 and 4,284,762; the disclosures of which areherein incorporated by reference. These hydrotalcite materials arecommercially available (see Example I). The presently preferredhydrotalcite has the chemical formula of Mg₄.5 Al₂ (OH)₁₃ CO₃.3.5H₂ O.

The impregnation of the support material with Pt and/or Pd (preferablyPt) can be carried out in any suitable manner. First, compounds of Ptand/or Pd are dissolved in a suitable solvent (preferably water) so asto prepare solutions of suitable concentration, generally containingfrom about 0.002 to about 0.20, preferably about 0.005 to about 0.05, gPt and/or Pd per cc of solution. Non-limiting examples of suitablecompounds of Pt and Pd are: PtCl₂, PtCl₄, PtBr₄, Pt(NH₃)₄ Cl₂, Pt(NH₃)₄(NO₃)₂ and the like; PdCl₂, PdCl₄, Pd(NH₃)₄ (NO₃)₂ and the like;preferably (at present) Pt(NH₃)₄ (NO₃)₂. The solution must have a pH ofat least about 5 (i.e., about 5 or higher), preferably about 6-8, so asto avoid damage to the structure of hydrotalcite. The support materialis then impregnated by soaking it in the solution of Pt and/or Pdcompounds; or (less preferably) the Pt and/or Pd containing solution issprayed onto the support material. The ratio of Pt and/or Pd solution tosupport material generally is such that the final catalyst containsabout 0.5 to about 5, preferably about 1 to about 3, weight-% Pt or Pd,more preferably Pt. When a solution contains both Pt and Pd compounds,the level of Pt and Pd generally is about 0.5 to about 5, preferablyabout 1 to about 3, weight percent (Pt+Pd).

In a preferred embodiment, a compound of iron is also present in theimpregnating solution (besides Pt and/or Pd). Non-limiting examples ofsuitable Fe compounds that can be used as solutes are Fe carboxylates,Fe(NO₃)₂, Fe(NO₃)₃, Fe(II) acetylacetonate (presently preferred), andthe like. Generally, the concentration of Fe (expressed as Fe metal) isin the range of from about 0.001 to about 0.2, preferably about 0.002 toabout 0.05, g Fe per cc solution (preferably aqueous or alcoholic). Theimpregnation of the support material with Pt and/or Pd and Fe can becarried out either by sequential impregnation (first Pt and/or Pd, thenFe) or by simultaneous impregnation in step (a) (using a solutioncontaining Pt and/or Pd compounds and at least one Fe compound).

When sequential impregnation is employed, the impregnation with asolution of at least one Fe compound is carried out after heating step(b) and before step (c). Thus, after step (b) an impregnating step (a*)with at least one dissolved Fe compound follows, and a subsequentheating step (b*) is carried out in substantially the same manner asstep (b). If preferred step (c) is also performed, step (c) is carriedout with the product obtained in step (b*). The ratio of Fe containingsolution to support material is such as to provide a level of about 0.1to about 4, preferably about 0.2-2, weight percent Fe.

Heating step (b) is generally carried out in an inert or oxidizingatmosphere, preferably a free oxygen containing gas atmosphere (such asair), generally at a temperature ranging from about 200° to about 400°C. for about 0.5-5 hours.

Reducing step (c) can be carried out in any suitable manner, preferablyat a temperature in the range of from about 50° to about 450° C., morepreferably from about 100° to about 300° C. Any reducing gas can beemployed, such as a gas comprising H₂, CO, gaseous hydrocarbons such asmethane, mixtures of the above, and the like. Preferably, a freehydrogen containing gas or a carbon monoxide containing gas, morepreferably a hydrogen gas stream (substantially pure), is employed.Reducing step (c) can be carried out for any suitable period of timesuitable to activate the calcined material obtained in step (b), or, ifapplicable, step (b*), preferably from about 0.5 to about 10 hours, morepreferably about 1-4 hours.

The process for oxidizing a carbon monoxide containing feed gas can becarried out at any suitable temperature and pressure conditions, for anysuitable length of time, at any suitable gas hourly space velocity, andany suitable volume ratio of CO and O₂. The reaction temperaturegenerally is in the range of from about -60° to about 400° C.,preferably from about 0° to about 100° C., more preferably from about20° to about 50° C. The pressure during the oxidation process generallyis in the range of from about 1 to about 2,000 psia, preferably fromabout 5 to about 40 psia. The volume ratio of CO to O₂ in the feed gascan range from about 1:100 to about 100:1, and preferably is in therange of from about 1:10 to about 10:1. The volume percentage of CO andthe volume percentage of O₂ in the feed gas can each be in the range offrom about 0.05 to about 50, preferably from about 0.05 to about 3. Thegas hourly space velocity (cc feed gas per cc catalyst per hour) can bein the range of from about 0.5 to about 10,000, preferably from about 1to about 1,000. It is understood that the calculation of the gas hourlyspace velocity is based on the volume of the active catalyst i.e., thehydrotalcite-supported Pt and/or Pd catalyst (optionally also containingiron oxide), excluding the volume occupied by any additional inertsupport material, such as a monolith, which may be present.

The feed gas can be formed in any suitable manner, e.g., by mixing CO,O₂ and, optionally, other gases such as CO₂, N₂, He and the like, suchas in a carbon dioxide laser cavity. Or the feed gas can be an exhaustgas from a combustion engine, or it can be air that is to be inhaled byhumans and contains undesirably high levels of toxic carbon monoxide,and the like. The feed gas can be contacted in any suitable vessel orapparatus, such as in a laser cavity or in an exhaust pipe of acombustion engine, or in a gas mask (used by humans), wherein the feedgas passes over the catalyst composition of this invention at theconditions described above. The CO oxidation process of this inventioncan be carried out in any suitable setting and for any purpose, e.g., torecombine CO and O₂ in CO₂ lasers, to oxidize CO contained in exhaustgases or in air, to make isotopically labeled CO₂ from CO and the ¹⁸ Oisotope, and the like.

The following examples are presented in further illustration of theinvention and are not to be construed as unduly limiting the scope ofthe invention.

EXAMPLE I

This example illustrates the preparation of hydrotalcite-supportedcatalysts.

The hydrotalcite support material (also referred to as hydrotalcite-likecompound) used as support was manufactured by Kyowa Chemical IndustryCompany, Ltd., Osaka, Japan, and was marketed by Mitsui and Company(U.S.A.), Inc., Houston, Tex. under the product designation DHT-4A.DHT-4A hydrotalcite was a basic aluminum hydroxide carbonate hydratehaving the chemical formula of Mg₄.5 Al₂ (OH)₁₃ CO₃.3.5H₂ O, a surfacearea (measured by N₂ adsorption in accordance with the BET method) of10±5 m² /g, a density of 2.1 g/cc, and a Mohs hardness of 2-2.5. Thehydrotalcite had a layered crystal structure with the following layersequence: OH⁻, (Mg²⁺, Al³⁺), OH⁻, (CO₃ ²⁻, H₂ O), wherein the distancebetween two successive Mg²⁺ layers was 7.63 angstroms and the distancebetween two successive OH⁻ layer, located above and below a (Mg²⁺, Al⁺³)layer, was 4.77 angstroms. The material lost about 0.3 weight-% whendried at 105° C. for 3 hours, and was a white powder (95.7% of which hada particle size of less than 1 micrometer).

Catalyst A was prepared by mixing 10 grams of DHT-4A hydrotalcite, whichhad been calcined in air at about 400° C. for about 5 hours, with about15 cc of an aqueous solution of Pt(NH₃)₆ (NO₃)₂, having a pH of about 7and containing 0.010 g Pt per cc solution. The thus-impregnated materialwas dried and calcined in air at 300° C. for 3 hours. The calcinedmaterial was ground to a powder and impregnated twice by mixing with0.23 grams of Fe(II) acetylacetonate (formula weight: 254), dissolved in30 cc methanol. The mixture was dried and calcined in air at 400° C. for3 hours. The Pt/Fe-impregnated hydrotalcite was allowed to cool in adesiccator, ground to a powder, mixed with enough water to make a thickpaste, dried, and sieved. A 10-20 mesh fraction was heated in air at400° C. for 2 hours, and then reduced in a stream of substantially purehydrogen gas at about 200° C. for about 2 hours. Catalyst A contained1.5 weight-% Pt and 0.5 weight-% Fe.

Catalyst B was prepared by mixing 10 grams of calcined DHT-4Ahydrotalcite with 15 cc of an aqueous solution of H₂ PtCl₆ having a pHof about 2 and containing 0.010 g Pt per cc solution. Thethus-impregnated material was dried and calcined in air at 300° C. for 3hours, ground to a powder, and impregnated with 10 cc of an aqueoussolution of Fe(NO₃)₃ containing 0.005 g Fe per cc solution. ThePt/Fe-impregnated hydrotalcite was calcined, cooled, ground, wetted withwater, dried, sieved, calcined and reduced in accordance with theprocedure for Catalyst A. Catalyst B also contained 1.5 weight-% Pt and0.5 weight-% Fe.

EXAMPLE II

The example illustrates tests for determining activity of thePt/Fe-promoted hydrotalcite catalysts for catalyzing the oxidation ofcarbon monoxide at low temperatures (to simulate catalytic recombinationof CO and O₂ in CO₂ lasers).

A gaseous feed blend comprising CO, O₂, He and N₂ was passed through aflow meter, a needle valve and a glass reactor tube in an upflowdirection. The glass reactor tube had an inner diameter of about 0.7 cmand generally contained a bed of about 0.5 cc (1 g) catalyst to betested. The temperature in the catalyst bed was measured by means of athermocouple inserted into the top of the catalyst bed. The content ofCO in the gaseous effluent from the reactor was determined by means of aBeckman Model 864 CO infrared analyzer, and CO conversion was determinedfrom these IR analyzer data.

All tests were carried out at about 26° C. and atmospheric pressureconditions. The feed rate of the gaseous feed stream was about 30cc/minute. The gas feed contained 1.2 volume-% CO, 0.6 volume-% O₂, 48.6volume-% N₂ and 49.7 volume-% He. Test results are summarized in TableI.

                  TABLE I                                                         ______________________________________                                                Run Time   % Conversion Conversion of CO                              Catalyst                                                                              (Hours)    of CO        in cc/minute                                  ______________________________________                                        A        2         65.8         0.23                                                   4         55.6         0.20                                                   6         48.7         0.17                                                   8         43.6         0.15                                                  10         40.2         0.14                                                  12         37.6         0.13                                                  14         35.9         0.13                                                  16         34.2         0.12                                          B       1-2        0            0                                             ______________________________________                                    

Test data in Table I show that the Pt/Fe/hydrotalcite catalyst, preparedby impregnation with neutral Pt(NH₃)₆ (NO₃)₂ (i.e., Catalyst A), wasquite active as a CO oxidation catalyst, whereas Catalyst B, prepared byimpregnation with acidic H₂ PtCl₆, was inactive as a CO oxidationcatalyst.

Additional tests showed that hydrotalcite was damaged by aqueoussolutions having a pH of 2-4 and was gradually dissolved in thesesolutions. On the other hand, no dissolution of hydrotalcite occured ina neutral aqueous solution (pH: 7). Based on these test results and thedata shown in Table I, it is concluded that the impregnating solutionscontaining Pt and/or Fe compounds must have a pH of at least about 5,i.e., about 5 or higher, preferably about 6-8.

Reasonable variations, modifications and adaptations for various usagesand conditions can be made within the scope of the disclosure and theappended claims, without departing from the scope of this invention.

That which is claimed is:
 1. A process for oxidizing carbon monoxidecomprising the step ofcontacting a gas mixture comprising CO and O₂ withan activated catalyst composition comprising hydrotalcite, platinummetal and iron oxide, under such reaction conditions as to at leastpartially convert said CO and O₂ to CO₂ ; wherein said activatedcatalyst composition has been prepared by a preparation processcomprising the steps of(a) contacting a support material comprisinghydrotalcite with a solution having a pH of at least about 5 andcomprising at least one dissolved platinum compound and at least onedissolved iron compound; (b) heating the material obtained in step (a)under such conditions as to substantially dry said material obtained instep (a) and to at least partially convert said at least one platinumcompound to at least one substrate selected from the group consisting ofplatinum oxides and platinum metal, and to at least partially convertsaid at least one iron compound to iron oxide; and (c) heating thematerial obtained in step (b) in a reducing gas atmosphere, under suchconditions as to form said activated catalyst composition.
 2. A processin accordance with claim 1, wherein said reducing gas atmosphere isselected from the group consisting of a free hydrogen containing gas anda carbon monoxide containing gas, and said conditions in step (c)comprise a temperature in the range of from about 50° to about 450° C.3. A process in accordance with claim 2, wherein step (c) is carried outin a stream of hydrogen gas at a temperature in the range of from about100° to about 300° C., for a period of time in the range of from about0.5 to about 10 hours.
 4. A process in accordance with claim 1, whereinsaid support material is a hydrotalcite having the chemical formula ofMg₄.5 Al₂ (OH)₁₃ CO₃.3.5H₂ O.
 5. A process in accordance with claim 1,wherein said catalyst composition comprises about 0.5 to about 5 weightpercent Pt and about 0.1 to about 4 weight percent Fe.
 6. A process inaccordance with claim 1, wherein heating step (b) is carried out at atemperature of about 200°-400° C.
 7. A process in accordance with claim6, wherein said heating step is carried out in a free oxygen containinggas atmosphere.
 8. A process in accordance with claim 1, wherein saidsolution used in step (a) is aqueous and has a pH of about 6-8.
 9. Aprocess in accordance with claim 1, wherein said reaction conditionscomprise a reaction temperature in the range of from about -60° C. toabout 400° C., a reaction pressure in the range of from about 1 to about2,000 psia and a volume ratio of CO to O₂ in the range of from about1:100 to about 100:1.
 10. A process in accordance with claim 1, whereinsaid reaction conditions comprise a temperature in the range of fromabout 0° C. to about 100° C., a reaction pressure in the range of fromabout 5 to about 40 psia, and a volume ratio of CO to O₂ in the range offrom about 1:10 to about 10:1.
 11. A process in accordance with claim 1,wherein said process is carried out in a cavity of a CO₂ laser so as torecombine CO and O₂ formed by dissociation of CO₂.
 12. A process inaccordance with claim 1, wherein in step (b) said at least one platinumcompound is substantially converted to at least one substance selectedfrom the group consisting of platinum oxides and platinum metal, andsaid at least one iron compound is substantially converted to ironoxide.
 13. A process in accordance with claim 1, wherein the pH of saidsolution used in step (a) is about
 7. 14. A process in accordance withclaim 5, wherein said catalyst composition comprises about 1 to about 3weight percent Pt and about 0.2 to about 2 weight percent Fe.
 15. Aprocess for oxidizing carbon monoxide comprising the step ofcontacting agas mixture comprising CO and O₂ with an activated catalyst compositioncomprising hydrotalcite, platinum metal and iron oxide, under suchreaction conditions as to at least partially convert said CO and O₂ toCO₂ ; wherein said activated catalyst composition has been prepared by apreparation process comprising the steps of(a) contacting a supportmaterial comprising hydrotalcite with a solution having a pH of at leastabout 5 and comprising at least one dissolved platinum compound; (b)heating the material obtained in step (a) under such conditions as tosubstantially dry said material obtained in step (a) and to at leastpartially convert said at least one platinum compound to at least onesubstance selected from the group consisting of platinum oxides andplatinum metal;(a*) impregnating the material obtained in step (b) witha solution comprising at least one dissolved iron compound; (b*) heatingthe material obtained in step (a*) under such conditions as tosubstantially dry said material obtained in step (a*) and to at leastpartially convert said at least one iron compound to iron oxide; and (c)heating the material obtained in step (b*) in a reducing gas atmosphere,under such conditions as to form said activated catalyst composition.16. A process in accordance with claim 15, wherein said reducing gasatmosphere is selected from the group consisting of a free hydrogencontaining gas and a carbon monoxide containing gas, and said conditionsin step (c) comprise a temperature in the range of from about 50° toabout 450° C.
 17. A process in accordance with claim 16, wherein step(c) is carried out in a stream of hydrogen gas at a temperature in therange of from about 100° to about 300° C., for a period of time in therange of from about 0.5 to about 10 hours.
 18. A process in accordancewith claim 15, wherein said support material is hydrotalcite having thechemical formula of Mg₄.5 Al₂ (OH)₁₃ CO₃.3.5H₂ O.
 19. A process inaccordance with claim 15, wherein said catalyst composition comprisesabout 0.5 to about 5 weight percent Pt and about 0.1 to about 4 weightpercent Fe.
 20. A process in accordance with claim 19, wherein saidcatalyst composition comprises about 1 to about 3 weight percent Pt andabout 0.2 to 2 weight percent Fe.
 21. A process in accordance with claim15, wherein said at least one dissolved iron compound used in step (a*)is Fe(II) acetylacetonate.
 22. A process in accordance with claim 15,wherein the pH of the solution used in step (a) is about 6-8.
 23. Aprocess in accordance with claim 15, wherein the pH of the solution usedin step (a) is about
 7. 24. A process in accordance with claim 15,wherein said at least one platinum compound is Pt(NH₃)₄ (NO₃)₂ and saidat least one iron compound is Fe(II) acetylacetonate.
 25. A process inaccordance with claim 15, wherein heating step (b) is carried out at atemperature of about 200°-400° C.
 26. A process in accordance with claim15, wherein heating step (b) is carried out in a free oxygen containinggas atmosphere.
 27. A process in accordance with claim 15, whereinheating step (b*) is carried out at a temperature of about 200°-400° C.28. A process in accordance with claim 15, wherein heating step (b*) iscarried out in a free oxygen containing gas atmosphere.
 29. A process inaccordance with claim 15, wherein in step (b) said at least one platinumcompound is substantially converted to at least one substance selectedfrom the group consisting of platinum oxides and platinum metal, and instep (b*) said at least one iron compound is substantially converted toiron oxide.
 30. A process in accordance with claim 15, wherein saidreaction conditions comprise a reaction temperature in the range of fromabout -60° C. to about 400° C., a reaction pressure in the range of fromabout 1 to about 2,000 psia and a volume ratio of CO and O₂ in the rangeof from about 1:100 to about 100:1.
 31. A process in accordance withclaim 15, wherein said reaction conditions comprise a temperature in therange of from about 0° C. to about 100° C., a reaction pressure in therange of from about 5 to about 40 psia, and a volume ratio of CO to O₂in the range of from about 1:10 to about 10:1.
 32. A process inaccordance with claim 15, wherein said process is carried out in acavity of a CO₂ laser so as to combine CO and O₂ formed by dissociationof CO₂.
 33. A process for oxidizing carbon monoxide comprising the stepofcontacting a gas mixture comprising CO and O₂ with an activatedcatalyst composition consisting essentially of hydrotalcite as supportmaterial, platinum metal and iron oxide, under such reaction conditionsas to at least partial convert said CO and O₂ to CO₂.
 34. A process inaccordance with claim 33, wherein said support material is ahydrotalcite having the chemical formula of Mg₄.5 Al₂ (OH)₁₃ CO₃.3.5H₂O.
 35. A process in accordance with claim 33, wherein said catalystcomposition contains about 0.5 to about 5 weight percent Pt and about0.1 to about 4 weight percent Fe.
 36. A process in accordance with claim33, wherein said catalyst composition contains about 1 to about 3 weightpercent Pt and about 0.2 to about 2 weight percent Fe.
 37. A process inaccordance with claim 33, wherein said reaction conditions comprise areaction temperature in the range of from about -60° C. to about 400°C., a reaction pressure in the range of from about 1 to about 2,000 psiaand a volume ratio of CO to O₂ in the range of from about 1:100 to about100:1.
 38. A process in accordance with claim 33, wherein said reactionconditions comprise a temperature in the range of from about 0° C. toabout 100° C., a reaction pressure in the range of from about 5 to about40 psia, and a volume ratio of CO to O₂ in the range of from about 1:10to about 10:1.
 39. A process in accordance with claim 33, wherein saidprocess is carried out in a cavity of a CO₂ laser so as to recombine COand O₂ formed by dissociation of CO₂.